The present invention relates generally to terahertz (THz) imaging systems, and more particularly to such imaging systems that can acquire images in real-time at video rates, e.g., by utilizing a coherent narrow-band THz illumination source.
The transparency of materials to radiation having frequencies in a range of about 0.1 to about 10 terahertz (THz) can be quite different than their transparency to radiation having frequencies in the visible range. For example, germanium, silicon, polytetrafluoroethylene plastic (Teflon), high and low-density polyethylene plastics (HDPE and LDPE), teeth, leaves, clothing and envelopes typically become more transparent at these terahertz frequencies. The increased transparency of many materials at terahertz frequencies has prompted the development of a variety of terahertz imaging systems. However, conventional imaging systems often obtain an image of an object by linearly scanning the object through a tightly focused THz beam—a practice that limits the image acquisition time to the mechanical scan rate of the system. With upper limits of hundreds of pixels per second for acquiring an image by mechanical scanning, in this approach, a complete image requires minutes to be acquired.
Alternatively, conventional terahertz imaging systems suitable for acquiring real-time images (e.g., 20 frames per second or more) typically employ an electro-optic crystal for frequency upconversion of THz pulses to optical pulses so that a CCD focal-plane camera can be employed for detecting the radiation and generating images. This arrangement, however, requires precise timing of optical and THz pulses, thus necessitating a scanning delay mechanism that adds to system complexity. Moreover, such systems are inherently broad-band (typically short THz pulses (e.g., less than 1 ps) are utilized that result in a bandwidth larger that 1 THz), and hence not suitable for applications that require coherent narrow-band illuminating radiation for frequency-sensitive imaging.
Accordingly, there is a need for an enhanced terahertz imaging system that can provide real-time, high resolution images.